在使用RecyclerView作为一个横向滑动控件时,有时对滑动后RecyclerView中的Item停留位置有要求,如希望Item居中等:
第一排为滑动后居中,第二排为滑动后左对齐
之前做这种需求需要自己去实现,在Support library 的24.2.0中增加了SnapHelper,可以满足我们的需求。
SnapHelper是一个抽象类,support library中只提供了一个继承类LinearSnapHelper ,可以使item居中。SnapHelper使用起来很简单:
SnapHelper snapHelper = new LinearSnapHelper();snapHelper.attachToRecyclerView(yourRecyclerView);
实现item 左对齐
LinearSnapHelper是居中操作,如何实现左对齐呢,我们可以继承LinearSnapHelper并覆盖其中的两个方法即可以实现:
1. calculateDistanceToFinalSnap
覆盖这个方法去设置目标View或者Container View滚动到指定的位置。当SnapHelper在fling结束时回调该方法去获取到达最终位置需要做的调整,即当recyclerview正常滑动结束后可能并不是在我们需要的位置,然后回调该方法,获取还需要调整的距离。
2. findSnapView
覆盖这个方法,可以设置最重需要调整的目标View。当SnapHelper需要开始snapping时回调该方法获取需要调整的target view。
我们看一下最中的实现代码:
public class StartSnapHelper extends LinearSnapHelper {
private OrientationHelper mVerticalHelper, mHorizontalHelper;
public StartSnapHelper() {
}
@Override
public void attachToRecyclerView(@Nullable RecyclerView recyclerView)
throws IllegalStateException {
super.attachToRecyclerView(recyclerView);
}
@Override
public int[] calculateDistanceToFinalSnap(@NonNull RecyclerView.LayoutManager layoutManager,
@NonNull View targetView) {
int[] out = new int[2];
if (layoutManager.canScrollHorizontally()) {
out[0] = distanceToStart(targetView, getHorizontalHelper(layoutManager));
} else {
out[0] = 0;
}
if (layoutManager.canScrollVertically()) {
out[1] = distanceToStart(targetView, getVerticalHelper(layoutManager));
} else {
out[1] = 0;
}
return out;
}
@Override
public View findSnapView(RecyclerView.LayoutManager layoutManager) {
if (layoutManager instanceof LinearLayoutManager) {
if (layoutManager.canScrollHorizontally()) {
return getStartView(layoutManager, getHorizontalHelper(layoutManager));
} else {
return getStartView(layoutManager, getVerticalHelper(layoutManager));
}
}
return super.findSnapView(layoutManager);
}
private int distanceToStart(View targetView, OrientationHelper helper) {
return helper.getDecoratedStart(targetView) - helper.getStartAfterPadding();
}
private View getStartView(RecyclerView.LayoutManager layoutManager,
OrientationHelper helper) {
if (layoutManager instanceof LinearLayoutManager) {
int firstChild = ((LinearLayoutManager) layoutManager).findFirstVisibleItemPosition();
boolean isLastItem = ((LinearLayoutManager) layoutManager)
.findLastCompletelyVisibleItemPosition()
== layoutManager.getItemCount() - 1;
if (firstChild == RecyclerView.NO_POSITION || isLastItem) {
return null;
}
View child = layoutManager.findViewByPosition(firstChild);
if (helper.getDecoratedEnd(child) >= helper.getDecoratedMeasurement(child) / 2
&& helper.getDecoratedEnd(child) > 0) {
return child;
} else {
if (((LinearLayoutManager) layoutManager).findLastCompletelyVisibleItemPosition()
== layoutManager.getItemCount() - 1) {
return null;
} else {
return layoutManager.findViewByPosition(firstChild + 1);
}
}
}
return super.findSnapView(layoutManager);
}
private OrientationHelper getVerticalHelper(RecyclerView.LayoutManager layoutManager) {
if (mVerticalHelper == null) {
mVerticalHelper = OrientationHelper.createVerticalHelper(layoutManager);
}
return mVerticalHelper;
}
private OrientationHelper getHorizontalHelper(RecyclerView.LayoutManager layoutManager) {
if (mHorizontalHelper == null) {
mHorizontalHelper = OrientationHelper.createHorizontalHelper(layoutManager);
}
return mHorizontalHelper;
}
}
使用方法和LinearSnapHelper是一样的。
感谢阅读,贴一下完整demo地址 https://github.com/MindorksOpenSource/SnapHelperExample
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